Off-Pump Coronary Artery Bypass (OPCAB) surgery: Implications for the anesthesiologist
Introduction
Two forces have driven cardiac surgeons to explore new approaches for coronary artery surgery. The first is a socioeconomic force: it is ubiquitous in medicine but applies particularly to the field of CABG surgery because this intervention, although beneficial, is so costly to the community. The second is of a competitive nature : the development of interventional cardiology with PTCA and stenting of coronary arteries has offered elegant and less invasive alternatives to surgery for a large percentage of patients with ischemic heart disease. In order to compete with these therapies CABG ought to become less costly and less invasive without losing its advantageous excellent long-term results.
The position of cardiopulmonary bypass (CPB) as being an essential part of coronary artery bypass grafting (CABG) surgery has been seriously challenged over the past few years. Sequential attempts to minimize or even circumvene the trauma associated with the use of CPB have ultimately led to new appoaches. From these, the OPCAB technique appears to be the most promising strategy. In the hands of experienced surgeons, up to 85 % of patients with multivessel ischemic heart disease can be completely revascularized without the use of CPB. While it is intuitively felt that such an approach will decrease morbidity and mortality, a scientific proof of this hypothesis is now due.
The successful completion of OPCAB surgery depends to a large extent on a skilled team in which the anesthesiologist plays an important role. Hemodynamic management of the surgically manipulated and dislocated heart as well as monitoring, prevention and treatment of myocardial ischemia and reperfusion are important objectives. Some patients will not tolerate OPCAB surgery despite optimal hemodynamic management. For these patients, mechanical circulatory support appears necessary and the use of minimally invasive assist devices offers an elegant alternative to classical CPB.
The goal of this report is to provide some guidelines for the management of patients undergoing straightforward OPCAB surgery and OPCAB surgery during VAD support with the novel Impella coaxial blood pump. We currently use OPCAB surgery in 85 % of our patients admitted for CABG. The hospital mortality is significantly lower than that predicted by the Euroscore. The number of grafts per patient is similar to our ‘pre-OPCAB era’.
General Anesthesia
To anesthesiologists the concept of "general anesthesia" covers several aspects that require close control in the pre- intra- and postoperative care of surgical patients. General Anesthesia consists not only of an adequate induction and maintenance of unconsciousness but also of the continuous titration to proper (i.e. adjusted to the type of surgery) levels of analgesia, amnesia, neuromuscular blockade and stress control.
For each of these separate pharmacodynamic goals the modern anesthesiologist uses a host of different drugs. Current developments in the field have produced pharmacological agents with favourable pharmacokinetic properties allowing a fast onset and offset of their effects.
For the anesthetic management of patients undergoing OPCAB surgery there is no reason to select "special" drugs. The only evolution that has occurred over the past few years is the gradual omission of the "high dose opioid" technique. The use of high dose opioids (analgesic drugs !) to obtain unconsciousness offers great hemodynamic stability at the expense of prolonged postoperative respiratory depression. The novel ultra-short acting opioid remifentanil is the only exception to this. Recent studies have shown that the addition of low doses of a hypnotic drug safeguards the patient from experiencing unintentional awareness. Although it is clear that several combinations of drugs are being used with success in a variety of medical centers we present our own technique here as a working example :
Premedication
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Alternative techniques include the use of remifentanil
- Induction at 1-2 µg/kg bolus
- Maintenance 0.5- 1 µg/kg/min continuous infusion
Volatile anesthetics are undoubtedly a fair choice since they have been shown to reduce the consequences of ischemia and reperfusion in experimental models.
The important matter is probably not which drugs are being used to control anesthesia but the way they are used (and this has been shown over and over again in our scientific literature for classic cardiac surgery/anesthesia)
Bispectral index monitoring to control the level of anesthesia can be used. BIS is maintained between 40 and 60. Any hemodynamic instability is treated with cardiovascular drugs and not by decreasing the dose of anesthetics.
Anticoagulation
In OPCAB surgery, the management no longer needs to focus on the prevention of CPB-induced coagulation activation with high-dose heparin. It is not clear however which doses of heparin should be employed to prevent thrombosis/coagulation during coronary artery occlusion and grafting . There is much institutional variability with regard to the exact requirements: we use Heparine 2 mg / kg to obtain an ACT of 400 seconds. The ACT is measured every 15 minutes and top up doses of heparin are administered when ACT is < 400 sec.
Hemodynamic management
The first major challenge during OPCAB surgery exists in managing the cardiovascular changes that occur with dislocation of the heart during exposition of the site of anastomosis. This event is often associated with kinking of systemic and pulmonary veins so that preload decreases acutely and cardiac performance drops suddenly if no appropriate precautions are taken. Surgical manipulation of the heart will also provoke arrhythmias and mechanical compression of the heart (mostly the right ventricle).
The second major challenge is in preventing the consequences of acute myocardial ischemia during coronary artery occlusion. The severity of myocardial ischemia and reperfusion will is related to the use of an intracoronary shunt. The decision to place an intracoronary shunt depends on surgical factors such as the luminal size of the coronary artery.
Cardiovascular stability
- Certain goals (more are less arbitrarily chosen) should be aimed at during the entire procedure. For our institution we persue the following hemodynamic conditions :
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The treatment of hemodynamic changes is based on an understanding of the pathophysiology involved. The principles are summarized below :
Preload
- Decreases due to obstructed Inferior and superior caval and Pulmonary venous return
- Preserve systemic and pulmonary venous return by elevation of the legs
- Trendelenburg position (may compromise SVC return)
- Administer Colloids to a Central venous pressure > 5-8 mmHg
- Obstruction of RV outflow tract due to compression also decreases LV preload (serial ventricular interdependence)
- Communicate with surgeon /reposition heart
Afterload
- As blood pressure falls with manipulation of the heart the coronary perfusion becomes compromised. Pharmacological increase of afterload may improve collateral perfusion. The decision to increase afterload depends on the preexisting ventricular function.
- In severely depressed ventricles low doses of inotropics are being administered together with vasopressors.
- In normal or near normal ventricular function we administer neosynephrine bolus per 50 µg IV
- Further arguments to use alpha 1 agonists are :
- They decrease venous capacitance (improve preload)
- They have a pharmacological preconditioning effect (G-protein coupled)
- They decrease venous capacitance (improve preload)
- In severely depressed ventricles low doses of inotropics are being administered together with vasopressors.
Rate - Rythm
- OPCAB surgery is demanding for the surgeon – a rigid control of sinus rhythm (predictable movements of the heart) is of primary importance. Sinus tachycardia, Ventricular extrasystoles and atrial fibrillation occur frequently.
- Potassium blood levels should be normalized
- We administer the following drugs in almost all patients as a prevention :
- Magnesium Sulfate 25 mg/kg
- Propafenone 1 mg / kg
- Other drugs that are being considered (but not routinely used) in order of importance :
- Xylocaine 1 mg / kg
- Diltiazem 0,25 mg /kg
- Esmolol 300µg/kg/min
- Lanoxin 7,5 µg/kg
- RAC bypass is often associated with bradycardia - AV junctional rhythm The decision to place atrial pacing leads should be considered.
- Tachycardia often first sign of ischemia
Contractility
- As myocardial consumption is directly related to the inotropic state positive inotropic stimulation are avoided if possible prior to the event of ischemia – grafting.
- In the postgrafting period positive inotropic drugs are used to treat myocardial stunning.
Preventing/reducing the consequences of myocardial ischemia and reperfusion
With routine CPB-supported CABG surgery, most surgeons use cardioplegia to temporarily protect the heart from ischemia during grafting. In OPCAB surgery, the grafting (and obligatory ischemia) takes place while the heart is expected to continue its normal function. All variables that determine myocardial oxygen demand should therefore be controlled and myocardial oxygen supply should be maximized through collateral supply if present. In practice the following methods are used at our institute :
- Intracoronary shunts are placed if the size of the grafted vessels permit
- Lidoflazine is administered at a rate of 1 mg/kg intravenously. A slow infusion over 30 minutes is started immediately after induction of anesthesia. Lidoflazine is a nucleoside-transport inhibitor that prevents adenosine from being washed out of the myocardium during reperfusion.
- Heart rate is maintained below 80 bpm (this is usually achieved with the above described anesthetic technique and the continuation of beta-blockade until the morning of surgery. If tachycardia is present prior to induction, beta-blockade is increased (metoprolol IV)
- Isosorbide dinitrate is used to treat increased wall tension during ischemia.
Monitoring
Much controversy persists on the type of monitoring that is adequate for OPCAB surgery. While we agree that the importance of a Thermodilution catheter in routine CABG should not be over-emphasized we plead for a reconsideration of its use in OPCAB surgery. Both ECG and 2-dimensional TEE have important limitations in monitoring ischemia during displacement of the heart during OPCABgrafting of the posterior and lateral walls of the heart.
Perhaps the most important hemodynamic variable in the monitoring of ischemia is the Left ventricular end-diastolic pressure. Some groups prefer to introduce a left atrial catheter while others (including our group) rely on the PcWP as a surrogate for LVEDP. Indeed LV stiffness is one of the earliest signs of myocardial ischemia.
2-dimensional TEE is useful in the assessment of regional wall motion in the pre- and postrevascularization period. During revascularization the visibility of the heart may be compromized due to loss of acoustic coupling.
